1. Field of the Invention
The present invention relates to processing and recycling of spent protective atmosphere gas contaminated with fouling organic decomposition byproduct materials during thermal processing.
2. Discussion of the Background
Protective atmosphere gas used in heat treatment or other thermal processing has long been discarded after use in industries where the components being treated are coated or bound with organic species. Examples include batch annealing of metal coated with residual rolling oils and reduction and/or sintering of metal powders or fibers combined with organic binders. The spent atmosphere gases, usually at low pressure, have been considered too prone to formation of fouling deposits to be compressed, let-alone otherwise purified and recycled. This is true even when total impurities are extremely low.
The inventors of the present invention have discovered that these fouling tendencies usually arise from the formation of poly-cyclic aromatic hydrocarbons (PAH's) during the thermal processing step. Most compounds of this type form solid deposits at standard conditions. These solids are prone to further thermal dehydrogenation, with an eventual tendency to form a range of adherent solids referred to as “varnish,” “tar,” or “coke.” These fouling tendencies are especially strong when gas mixtures containing these compounds are compressed, for two reasons. First, increasing the total pressure of the mixture increases the thermodynamic tendency towards solid formation during interstage cooling, as the partial pressure of the contaminants is increased. And second, the high temperatures experienced in many compression cycles can accelerate the formation of thermal decomposition products such as varnish, tar and coke. If these are formed in critical moving parts, such as compressor valves, then malfunction will be greatly accelerated.